The present invention relates generally to voltage regulators and in particular to a temperature compensated voltage regulator for photovoltaic solar cell charging systems.
The voltage of a fully charged battery varies with its temperature, being a higher value at lower temperatures. The normal full-charge voltage of 14.4 volts (12 volt lead-acid battery) is only "correct" at a battery temperature of 25.degree. C. Temperature compensation is generally employed in sophisticated battery charging systems. The need for such a feature increases when temperature extremes are anticipated or where battery charging requirements become particularly dependent upon narrow changes in the battery temperature. In a conventional lead-acid cell, the charging characteristics are not significantly altered by small temperature changes, however, the important characteristics such as the maximum charging voltage, referred to in the art as finish voltage is temperature related. As the battery temperature is increased, the battery electrolyte expands resulting in a lower specific gravity. This change is equal to about 1 point (0.001) in specific gravity for every 3.degree. F. change in temperature. The battery temperature also affects the diffusion rate of the electrolyte and the internal resistance of the cell. These factors combine to influence the cell's electrical capacity and correspondingly affect the charging characteristics and correspondingly affect the charging characteristics of the battery. Exceeding the finished voltage, that is exceeding the full charge voltage, results in electrolyte gassing and its depletion. This, in turn, will result in battery deterioration. Gassing will occur at relatively small levels of over charging potential. These problems are typically avoided by providing a charging regulator circuit which compensates the maximum or finish voltage in response to the battery or ambient temperature. This feature is particularly emphasized in remote area applications where the storage batteries are not readily subject to inspection or, as mentioned heretofore, in applications where extreme temperature fluctuations are anticipated. A prime example of these conditions occur in space applications. In U.S. Pat. No. 3,740,636, for example, a temperature responsive portion of the charging regulator shunts the photovoltaic charging power from the storage batteries when a preselected maximum temperature is achieved.
Further applications which stress the importance of the temperature compensated charging regulation are charging systems for storage batteries which have peculiar temperature dependencies. One such storage battery is the nickel cadmium system. An exemplary charging system of a nickel cadmium battery may be found in U.S. Pat. No. 3,940,679. The primary concern of these systems is thermal runaway, a characteristic of nickel cadmium batteries. When the temperature of a nickel cadmium battery exceeds about 160.degree. F., the battery voltage decreases, permitting increased charging current to be drawn, in turn heating the battery further. A temperature responsive cut-off switch is conventionally employed in these systems. The present inventions would be operative to avoid the charging problems encountered in such thermal runaway systems not by temperature over-ride means, however, but by automatically adjusting the regulator's reference potential in response to a monitored temperature change. A voltage regulation of photovoltaic solar cell charging of storage batteries differs from constant current or constant voltage charging of same inasmuch as a photovoltaic solar cell array is subject to wide variances in power output, dependent upon solar insolance. The photovoltaic array/battery system is designed based upon average insulation values. The charging system is accordingly subject to wide variances in both current and voltage. A conventional regulator for a photovoltaic system would be primarily directed to prevention of over voltage charging of the storage batteries. In contrast to such systems, the present invention provides for temperature compensation of the maximum charging voltage providing increased protection of the storage battery systems.